As an example of an application of a filter, a radio-communication apparatus is discussed. For a radio-communication apparatus, both a high-speed data communication and a multi-terminal connection are required. Therefore, there can be a case, such that one terminal performs a high-speed communication by using a broad-band under a condition in which there are a small number of connection terminals, or a case, such that one terminal performs a data communication by using a narrow-band under a condition in which there are a large number of connection terminals.
In case of a low-pass filter, resistance of resistors or capacitance or capacitors in the low-pass filter is varied in order to vary the cut-off frequency. When the cut-off frequency is varied to be higher, a radio communication circuit can employ broader band. On the other hand, when the cut-off frequency is varied to be lower, a radio communication circuit can employ narrower band.
Also, a filter is applied to, not only a radio-communication apparatus, but also various kinds of apparatuses. As an example indirectly related to an embodiment illustrated hereinafter, a technology for switching a filter characteristic of a filter circuit to be applied to a DAC (Digital Analogue Converter), to which a Delta Sigma modulating method is used (refer to “Rigorous Analysis of Delta-Sigma Modulators for Fractional-N PLL Frequency Synthesis” (Transactions on Circuits and Systems), vol. 51, pp 1148-1162, June 2004), is illustrated.
With regard to the “D/A converting apparatus” described in Japanese patent laid-open publication No. 2003-273740, a technology for reducing noises caused by a limit cycle even at a time of zero input, by detecting a timing of the zero input signal and by switching the gain of LPH according thereto, is discussed.
Also, with regard to “a variable resistance circuit and D/A converter” described in Japanese patent laid-open publication No. 2001-77694, in relation to a D/A converter comprising soft mute function for detecting zero data, a technology for gradually reducing a feedback resistance at the output terminal of a LPF is discussed.